This invention relates generally to ammonium nitrate. More particularly, this invention relates to phase stabilized ammonium nitrate and a method of making the same.
It is well known to protect a vehicle occupant using a cushion or bag, e.g., an xe2x80x9cairbag cushion,xe2x80x9d that is inflated or expanded with gas when the vehicle encounters sudden deceleration, such as in the event of a collision. In such systems, the airbag cushion is normally housed in an uninflated and folded condition to minimize space requirements. Upon actuation of the system, the cushion begins to be inflated, in a matter of no more than a few milliseconds, with gas produced or supplied by a device commonly referred to as an xe2x80x9cinflator.xe2x80x9d
While many types of inflator devices have been disclosed in the art for use in the inflating of one or more inflatable restraint system airbag cushions, inflator devices which rely on the combustion of a pyrotechnic, fuel and oxidizer combination or other form of gas generant to produce or at least in part form the inflation gas issuing forth therefrom have been commonly employed in conjunction with vehicular inflatable restraint airbag cushions.
Sodium azide has been a commonly accepted and used gas generating material. While the use of sodium azide and certain other azide-based gas generant materials meets current industry specifications, guidelines and standards, such use may involve or raise potential concerns such as involving handling, supply and disposal of such materials.
The development of safe gas generant material alternatives to sodium azide for commercial application in inflatable restraint systems commonly involves the oftentimes conflicting goals or objectives of increasing the gas output of the gas generant material while reducing or minimizing the costs associated with the gas generant material, including the costs associated with ingredients and the processing thereof.
The incorporation and use of ammonium nitrate as an oxidizer in such gas generant formulations has been found to be one generally cost-effective approach for exceeding the current state of the art gas generant formulation gas yield of about 3 moles of gas per 100 grams of gas generant formulation. In particular, ammonium nitrate is relatively inexpensive and, when burned with guanidine nitrate fuel, generally combusts to all gaseous species resulting in gas yields approaching 4 moles of gas per 100 grams of material.
Unfortunately, the general incorporation and use of ammonium nitrate in pyrotechnic gas generant formulations has generally been subject to certain difficulties. For example, ammonium nitrate-containing pyrotechnic gas generant formulations have commonly been subject to one or more of the following shortcomings: low bum rates, burn rates exhibiting a high sensitivity to pressure, as well as to phase or other changes in crystalline structure such as may be associated with volumetric expansion such as may occur during temperature cycling over the normally expected or anticipated range of storage conditions, e.g., temperatures of about xe2x88x9240xc2x0 C. to about 110xc2x0 C. Such changes of form or structure may result in physical degradation of such gas generant formulation forms such as when such gas generant formulation has been shaped or formed into tablets, wafers or other selected shape or form. Further, such changes, even when relatively minute, can strongly influence the physical properties of a corresponding gas generant material and, in turn, strongly affect the burn rate of the generant material. Unless checked, such changes in ammonium nitrate structure may result in such performance variations in the gas generant materials incorporating such ammonium nitrate as to render such gas generant materials unacceptable for typical inflatable restraint system applications.
In view thereof, efforts have been directed to minimizing or eliminating such volume expansion during normal temperature cycling and the effects thereof. It has been found that the incorporation of a transition metal diammine dinitrate such as copper diammine dinitrate, nickel diammine dinitrate or zinc diammine dinitrate, for example, in ammonium nitrate, can serve to phase stabilize the mixture and minimize or eliminate volumetric expansion during normal temperature cycling. Further, ammonium nitrate stabilized with such transition metal diammine dinitrates are typically advantageously less hygroscopic than ammonium nitrate phase stabilized by other methods and the use of such transition metal dianunine dinitrates has also been found to result in combustion products which form a more easily filterable clinker.
A traditional method for the incorporation of a quantity of such metal diammine dinitrate into ammonium nitrate is outlined in U.S. Pat. No. 5,063,036. In general accordance therewith, a metal oxide (such as cupric oxide) is reacted with ammonium nitrate according to the following chemical reaction:
CuO+2NH4NO3xe2x86x92Cu(NH3)2(NO3)2+H2Oxe2x80x83xe2x80x83(1)
Such reaction occurs at elevated temperatures (e.g., temperatures in excess of 140xc2x0 C.) either in a solid state or in an ammonium nitrate melt. The rate of such solid state reaction is generally dependent on the processing temperature. However, even under normal processing conditions such a reaction process would normally take several hours to complete. Moreover, temperatures high enough to complete the reaction in a reasonable amount of time are typically high enough to cause safety concerns over self-decomposition of the ammonium nitrate and such as may possibly be accompanied by explosion. Performance of the reaction in an ammonium nitrate melt generally increases the reaction rate but such processing generally requires specialized equipment and techniques in order to efficiently convert the molten mixture into desirably easy to handle solid free-flowing granules. Further, processing temperatures required to melt such ammonium nitrate/metal oxide mixtures are also generally great enough to pose safety concerns.
An alternative reaction process proposed by Hommel et al. in U.S. Pat. No. 4,925,600 is to combine a metal nitrate with ammonia in aqueous solution to form the corresponding metal tetrammine nitrate (e.g., Cu(NH3)4(NO3)2). The metal tetrammine nitrate complex is then in turn isolated, dried, and mixed with solid ammonium nitrate and metal nitrate. This mixture can then be melted, atomized, and cooled to a granular form. In the melt, the following reaction occurs:
Cu(NH3)4(NO3)2+Cu(NO3)2+NH4NO3(excess)xe2x86x922Cu(NH3)2(NO3)2+NH4NO3(excess)xe2x80x83xe2x80x83(2)
The mixture of ammonium nitrate and metal tetrammine nitrate complex advantageously forms a eutectic mixture that melts at a lower temperature than a corresponding mixture of ammonium nitrate and the metal oxide. While such lower melting point increases the thermal safety margin associated with the reaction, such processing generally requires an extra reaction or processing step to prepare and isolate the metal tetrammine nitrate complex. Further, the handling and use of these complexes may also raise various safety concerns. As a result, such large scale production is generally not practical. Further, since the final reaction is performed in a melt, specialized equipment and techniques to efficiently convert the molten mixture into solid free-flowing granules is typically required.
Thus, there is a need and a demand for a method of making ammonium nitrate phase stabilized via the presence of a selected metal diammine dinitrate and which method avoids undesirably high processing temperatures, i.e., processing temperatures which are undesirably near the decomposition temperature of ammonium nitrate, and which avoids isolation of metal tetrammine nitrate complexes such as may raise shipping and handling concerns.
A general object of the invention is to provide an improved method of making phase stabilized ammonium nitrate as well as an improved resulting phase stabilized ammonium nitrate.
The general object of the invention can be attained, at least in part, through a method which includes drying and heat treating an aqueous slurry containing ammonium nitrate and a combination of at least one transition metal nitrate and an ammonia source to form a phase stabilized ammonium nitrate. In the slurry, the ammonia source is present in at least a stoichiometric amount relative to the at least one transition metal nitrate for formation of a corresponding transition metal diammine dinitrate.
The prior art generally fails to provide a method of making phase stabilized ammonium nitrate, particularly ammonium nitrate phase stabilized via the inclusion of a suitable phase stabilizing amount or proportion of a desired metal diammine dinitrate. Further, the prior art generally fails to provide a correspondingly phase stabilized ammonium nitrate such as retains properties or characteristics, such as relating to crush strength and density, to as great an extent as may be desired when subjected to temperature cycling such as can be expected in various intended applications.
The invention, in accordance with an alternative preferred embodiment of the invention, further comprehends a method of making phase stabilized ammonium nitrate, which method includes:
combining, in an aqueous slurry, at least a nitrate of at least one transition metal selected from the group of copper, zinc, nickel and combinations thereof and an ammonia source, in at least a stoichiometric amount relative to the transition metal nitrate for formation of a corresponding transition metal diammine dinitrate, to form a first precursor;
forming a second precursor to the phase stabilized ammonium nitrate, the second precursor being in the form of a slurry containing the first precursor and ammonium nitrate,
drying the second precursor to form a third precursor to the phase stabilized ammonium nitrate, the third precursor having the form of a powder, and
heat treating the third precursor to form a phase stabilized ammonium nitrate comprising ammonium nitrate containing at least about 1 wt. % of a diammine dinitrate of the at least one transition metal.
The invention, in accordance with another alternative preferred embodiment of the invention, still further comprehends a phase stabilized ammonium nitrate. In particular, such phase stabilized ammonium nitrate is desirably made by drying and heat treating an aqueous slurry containing ammonium nitrate and a combination of at least one transition metal nitrate and at least one ammonia source, present in at least a stoichiometric amount relative to the at least one transition metal nitrate for formation of a corresponding transition metal diammine dinitrate.
Other objects and advantages will be apparent to those skilled in the art from the following detailed description taken in conjunction with the appended claims and drawings.